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LM35C Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM35C
Description  Precision Centigrade Temperature Sensors
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com

LM35C Datasheet(HTML) 5 Page - National Semiconductor (TI)

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The LM35 can be applied easily in the same way as other
integrated-circuit temperature sensors It can be glued or
cemented to a surface and its temperature will be within
about 001 C of the surface temperature
This presumes that the ambient air temperature is almost
the same as the surface temperature if the air temperature
were much higher or lower than the surface temperature
the actual temperature of the LM35 die would be at an inter-
mediate temperature between the surface temperature and
the air temperature This is expecially true for the TO-92
plastic package where the copper leads are the principal
thermal path to carry heat into the device so its tempera-
ture might be closer to the air temperature than to the sur-
face temperature
To minimize this problem be sure that the wiring to the
LM35 as it leaves the device is held at the same tempera-
ture as the surface of interest The easiest way to do this is
to cover up these wires with a bead of epoxy which will
insure that the leads and wires are all at the same tempera-
ture as the surface and that the LM35 die’s temperature will
not be affected by the air temperature
The TO-46 metal package can also be soldered to a metal
surface or pipe without damage Of course in that case the
Vb terminal of the circuit will be grounded to that metal
Alternatively the LM35 can be mounted inside a sealed-end
metal tube and can then be dipped into a bath or screwed
into a threaded hole in a tank As with any IC the LM35 and
accompanying wiring and circuits must be kept insulated
and dry to avoid leakage and corrosion This is especially
true if the circuit may operate at cold temperatures where
condensation can occur Printed-circuit coatings and var-
nishes such as Humiseal and epoxy paints or dips are often
used to insure that moisture cannot corrode the LM35 or its
These devices are sometimes soldered to a small light-
weight heat fin to decrease the thermal time constant and
speed up the response in slowly-moving air On the other
hand a small thermal mass may be added to the sensor to
give the steadiest reading despite small deviations in the air
Temperature Rise of LM35 Due To Self-heating (Thermal Resistance)
no heat sink
small heat fin
no heat sink
small heat fin
no heat sink
small heat fin
no heat sink
small heat fin
Still air
400 CW
100 CW
180 CW
140 CW
220 CW
110 CW
85 CW
60 CW
Moving air
100 CW
40 CW
90 CW
70 CW
105 CW
90 CW
25 CW
40 CW
Still oil
100 CW
40 CW
90 CW
70 CW
Stirred oil
50 CW
30 CW
45 CW
40 CW
(Clamped to metal
Infinite heat sink)
(24 CW)
(55 CW)
(23 CW)
Wakefield type 201 or 1
disc of 0020
sheet brass soldered to case or similar
TO-92 and SO-8 packages glued and leads soldered to 1
square of
printed circuit board with 2 oz foil or similar
Typical Applications (Continued)
TLH5516 – 19
FIGURE 3 LM35 with Decoupling from Capacitive Load
TLH5516 – 20
FIGURE 4 LM35 with R-C Damper
Like most micropower circuits the LM35 has a limited ability
to drive heavy capacitive loads The LM35 by itself is able to
drive 50 pf without special precautions If heavier loads are
anticipated it is easy to isolate or decouple the load with a
resistor see
Figure 3 Or you can improve the tolerance of
capacitance with a series R-C damper from output to
ground see
Figure 4
When the LM35 is applied with a 200X load resistor as
shown in
Figure 5 6 or 8 it is relatively immune to wiring
capacitance because the capacitance forms a bypass from
ground to input not on the output However as with any
linear circuit connected to wires in a hostile environment its
performance can be affected adversely by intense electro-
magnetic sources such as relays radio transmitters motors
with arcing brushes SCR transients etc as its wiring can
act as a receiving antenna and its internal junctions can act
as rectifiers For best results in such cases a bypass capac-
itor from VIN to ground and a series R-C damper such as
75X in series with 02 or 1 mF from output to ground are
often useful These are shown in
Figures 13 14 and 16

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